# Broken crankshaft !?

### Help Support HMEM:

#### Rains

##### Member
I think, on the one hand, it is likely that the quality of the material, due to the material is not hard enough, or too hard, does not meet the application of the material, it is easy to bend, deformation, and so on. I once encountered this problem, and later replaced the product with better materials, and only later did not break.

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#### Rains

##### Member
I wonder if you have solved this problem now

#### Steamchick

##### Well-Known Member
Hi Tony Steam Hobb: your comment: "Mechanicboy suggested an undercut of the large diameter journal to create a smooth radius, I have done this when cutting screws for threading (when running threads to the head) and have wondered if the reduced diameter weakens the part at the head? So does the smooth radius spread out the stresses and negates the smaller dimension? ": As I have not spotted a reply from Mechanicboy yet - and I'm sure he'll help and correct me where I go wrong... - here's my understanding of what stress raisers/reducers do:
To look at a "sharp cornered journal" of 10mm diameter, on a shaft of 12mm diameter: Assuming the "sharp corner" is 0.25mm radius: The stress raiser would be extracted from a standard table using D/d, and r/d, where D = Larger shaft diameter, d is smaller shaft diameter, r = radius of tool/corner.
So D/d = 12/10 = 1.2, and r/d = 0.0025
For Bending: stress concentration factor (SCF) is something like 2.25~2.3.
For torsion, the SCF is something like 1.9.
And CSA ( area that is stressed = Pi x d-squared /4) = 78.5sq.mm.
Now if we undercut the corner by 1mm to make a radius of r=0.5mm, and d = 9mm, r/d becomes 0.0556, D/d becomes 1.33.
This moves us along the graphs of SCF values so we get:
Bending - SCF = 1.9, Torsion SCF becomes 1.65. So the SCF reduction is about 17.4%, Torsion SCF reduction about 13%.
The CSA of 63.6 sq.mm has an effect of increasing the stress by about 25%.
However, if you repeat the sums with a root diameter of 9.5mm, by taking the 0.5mm radius into the side wall a touch, then the D/d becomes 1.26, and r/d becomes 0.0526, which are small changes, yet the stress increase from the 10dia to 9,5dia is only about 11%.
Therefore there can be a selection of dimensions where the stress is increased - by reducing the diameter locally to permit a stress-reducing corner radius. And there is a selection of dimensions where the stress is reduced by choosing a significant change of SCF reduction against the stress increase of reducing the root diameter.
Correct selection of dimensions - by such calculations - are what make a good bit of Engineering when designing parts.
Ignorance is bliss, but sharing knowledge and understanding is (IMHO) simply beautiful and leads to more long-lived happiness. (Fewer failures).
"Here endeth today's lesson".
I hope it helps?

#### TonySteamHobby

##### Member
Yes that does help. I randomly picked some diameters and various radii (to make something which looked like Minh’s crank) and then modeled them in Freecad FEA. I was getting various results, but not what I expected. I was mostly expecting the stresses to move away from the radius but it did not…As I mentioned, I’m not formally trained in mechanical engineering and I “learn as required”. That’s why I visit the forum as my first order of business daily. Minh’s question has opened up a whole new area of learning for me!

Unfortunately, Freecad only let me model the bending stress and not the torsional stress, as seen above. Back to the drawing board…